Digestibility of fats depends on several factors which may be grouped as: (1) mechanical, e. g., melting point which determines the rate of gastric discharge1 and, to a considerable extent, the degree of emulsification; and (2) chemical which determines the character of the products of digestion and the rate of hydrolysis, certain esters of fatty acids, e. g., cetyl palmitate,2 being attacked very slowly by pancreatic lipase. There is strong evidence for the belief that unchanged esters in a finely emulsified form can not be absorbed by the intestinal mucosa.3 The facts presented here support this view and agree with the thesis of Terroine4 that absorption of fats is limited by the rapidity of hydrolysis.Two dogs were used for each experiment. The fats employed were added to a basal ration of lean beef, cracker crumbs, and agar. The ethyl palmitate used was neutral; melting point 24° saponification number 198 (theoretical 197.6). Glycerol palmitate was prepared by heating glycerol and palmitic acid togeth...